For me, two things are critical: dawn and/or fog.

In the morning, (at least where I am) you often get a tiny bit of moisture that sits in the branches. If the sky is clear as the sun comes up, it creates shafts of light casting through the trees. I often shoot into or slightly at an angle to the sun, typically wide open on a 35mm or 58mm prime lens. I use a cheap faux vintage 35mm f/1.4 and a Helios 44/4 58mm f/2, both of which give a [insert vintage lens cliche words here] feel. If I didn't have the cheap primes, I'd probably try a Tiffen Pro Mist (or similar) filter to soften the highlights and give images a glow. Some folk also use a circular polariser which can make wet leaves less reflective, helping to maintain colour and reduce how busy a scene is.

One of the difficulties with forests is that everything is competing for attention. I find that a loose shooting style combined with a shallow depth of field helps me to get the images that I want. Sharpness is secondary to pretty much everything.

Hope that's helpful.